TM59 and domestic overheating: Practical guidance on cooling UK homes for installers

TL;DR

• TM59 2026 is the standard, prescriptive method for assessing overheating risk in dwellings at design stage, using dynamic thermal modelling.
• It uses four criteria (a–d) and a three‑stage modelling strategy that prioritises passive measures before mechanical ventilation and cooling.
• Assessments must use specific CIBSE Design Summer Year (DSY1) 2050s, high‑emissions weather files as a minimum, with options to test more extreme scenarios.
• Passive cooling (shading, natural ventilation, thermal mass, layout) is the first line of defence; AC is only justified when these can’t deliver safe comfort.
• Bedrooms now have an updated night‑time criterion based on sleep research, with fixed thresholds and limits on the number of hot nights.
• Many domestic AC units use higher‑GWP refrigerants, and newer “low‑carbon” products often use flammable refrigerants like R290, which demand specialist training.
• Installers who understand TM59, passive cooling and low‑carbon refrigerants will be best placed as demand for domestic cooling grows.
• Logic4training offers regulated F-Gas training for air-conditioning, refrigeration and heat pump to help engineers install and maintain systems safely and competently.

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What is TM59?

TM59 is CIBSE’s design‑stage methodology for assessing domestic overheating in UK homes using dynamic thermal modelling.

It provides clear criteria and weather file requirements so designers can test whether new dwellings and major refurbishments are at risk of overheating before anything is built, with a strong emphasis on passive cooling and good ventilation.

Rather than prescribing exact solutions, TM59 sets the framework that shapes decisions on shading, window openings, mechanical ventilation and, only where necessary, cooling options such as air conditioning and heat pumps.

For installers, understanding TM59 helps them align their cooling and heat pump work with overheating guidance, making homes more comfortable, low‑carbon and compliant as UK summers get hotter.

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Why UK homes are overheating more

UK homes are not traditionally air‑conditioned, and modern designs have made overheating more likely. Highly insulated, airtight dwellings with big areas of glazing, lightweight construction and limited window opening can trap heat, particularly in flats and single‑aspect apartments.

The document notes that increased urbanisation, changing dwelling typologies and higher internal gains (appliances, lighting, hot water systems) all amplify summertime temperatures. Homes that overheat can cause discomfort, stress, adverse health impacts and, in the worst cases, lead to litigation or costly retrofit programmes to fix the problem after residents complain.

TM59 explicitly links overheating to the need for climate‑resilient design. It highlights that mechanical cooling can increase bills and carbon emissions, and intensify local urban heat islands, so it encourages robust passive design as the default.

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TM59 2026: What’s changed and why it matters

Purpose and scope

CIBSE’s TM59 is aimed at designers, modellers and building services professionals working on new homes and major refurbishments, including flats, houses, houses in multiple occupation (HMOs), sheltered accommodation, care homes, student blocks and bedrooms in hotels and hospitals.

The 2026 edition updates the 2017 version based on industry feedback, academic research and new climate projections. Key updates include:

• A revised bedroom night‑time criterion based on large‑scale sleep comfort research.
• Clarified treatment of ceiling fans, including when temperature thresholds can be “uplifted”.
• Updated weather file requirements aligned with the latest CIBSE future weather data.
• A three‑stage modelling strategy to deal with window opening constraints and staged introduction of mechanical systems.
• An added home office profile, reflecting increased home working.

For Logic4training learners involved in design, installation or technical support, basic familiarity with these concepts means they can speak the same language as consultants, housing providers and local authorities using TM59 for compliance and good practice.

The four TM59 criteria (a–d)

TM59 defines four overheating criteria, each with clear pass/fail outcomes.

Criterion a: Oredominantly naturally ventilated spaces

Applies to living rooms, kitchens, home offices and bedrooms when people are awake and able to adjust clothing, shading, windows and ceiling fans.

It limits the number of occupied hours between May and September when operative temperature exceeds an adaptive comfort threshold by at least 1 K, using Category I (more thermally sensitive occupants) or Category II thresholds.

Criterion b: Bedrooms when people are asleep

Applies to both naturally ventilated and mechanically ventilated/cooled bedrooms.

It sets a fixed night‑time temperature threshold (26°C for Category I, 27°C for Category II) and limits the number of nights between May and September when the mean operative temperature during assumed sleeping hours (23:00-08:00) exceeds that threshold to a maximum of four.

Criterion c: Predominantly mechanically ventilated/cooled spaces

Applies where spaces rely mainly on mechanical ventilation or active cooling during occupied hours.

It requires the operative temperature to stay at or below 26°C for at least 97% of occupied hours between May and September (i.e. no more than 3% of time above 26°C).

Criterion d: Communal circulation areas

Covers corridors, stairwells, lift lobbies and similar shared spaces in multi‑dwelling buildings.

It sets a fixed threshold of 28°C, allowing exceedance in up to 3% of occupied hours between May and September.

Ceiling fans can justify raising thresholds in Criteria a and c by defined amounts, but not for sleeping comfort (Criterion b) or communal areas (Criterion d).

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The three‑stage TM59 modelling strategy

A major feature of the 2026 TM59 is the three‑stage modelling strategy, which enforces a “passive first” hierarchy and handles site‑specific constraints such as noise, pollution and security.

Stage 1: Unconstrained natural ventilation, passive only

Stage 1 models all sampled dwellings assuming:

• Only passive heat mitigation measures (shading, ventilation, layout, thermal mass) are used.
• Site‑specific noise, security or air‑quality constraints on window opening are ignored.
• Background mechanical ventilation like MEV or MVHR with summer bypass may be included to meet whole‑house air requirements, but no mechanical cooling.

Spaces are assessed using Criterion a (awake) and Criterion b (bedrooms) for predominantly naturally ventilated areas, plus Criterion d for communal areas. If any space fails, the design must be improved with passive measures (and ceiling fans if appropriate) until Stage 1 passes for all spaces.

For many UK homes without serious external constraints, Stage 1 alone can demonstrate that a passive design is sufficient, avoiding the need for AC.

Stage 2: Ventilation constraints and mechanical ventilation

Stage 2 introduces real‑world constraints on natural ventilation, such as:

• External noise that prevents night‑time window opening.
• Security concerns affecting ground‑floor or easily accessible bedrooms.
• Poor air quality that restricts opening at certain times.

All passive measures from Stage 1 are retained, and additional passive measures or enhanced mechanical ventilation (but still no cooling) can be added where needed.

Spaces that can still use windows for at least half of the occupied hours are treated as predominantly naturally ventilated and assessed with Criteria a and b. Where openings must remain closed for more than half the time and mechanical systems are used, Criteria b and c apply.

The goal is to secure a Stage 2 pass without AC wherever possible, using intelligent design plus mechanical ventilation when necessary.

Stage 3: Mechanical cooling as a last resort

Stage 3 is only used where Stage 2 cannot achieve a pass, even with enhanced passive and ventilative measures.

At this stage:

• All passive measures and constraints from previous stages remain.
• Mechanical ventilation with active cooling (e.g. AC, chilled beams, cooled supply air) may be introduced in specific spaces.
• Spaces are assessed using Criteria b and c, with Criterion d still applying to communal areas.

A scheme only achieves overall compliance if every habitable space in every sampled dwelling passes Stage 1 and then either Stage 2 or Stage 3, and communal areas pass Criterion d.

For Logic4training trainees, understanding this staged logic helps them see where AC genuinely belongs in a design and where passive or mechanical ventilation solutions should be prioritised instead of defaulting to cooling.

Weather files: What TM59 now requires

The supporting “weather file requirements” guidance sits alongside TM59 and sets out exactly which CIBSE weather files must be used.

Minimum requirement for TM59 assessments

TM59 requires assessments to use the latest Design Summer Year DSY1 file relevant to the site location, for the 2050s, high‑emission (RCP8.5), 50th percentile scenario. These files are labelled: ZoneReferenceDSY1 2050s HIGH 50 CIBSE v1.1

This ensures designs are tested against a realistic mid‑range projection of hotter future summers under a continuation of current global emissions trends.

Alternative and more extreme scenarios

CIBSE’s 2025 weather data set also provides:

• DSY2 files representing the most intense heat events.
• DSY3 files representing the longest heat events.
• Test Reference Years (TRYs) for typical annual energy modelling.
• Multiple emission scenarios (RCP2.6, 4.5, 8.5) and probability percentiles (10th, 50th, 90th).

TM59 encourages designers and clients to test more extreme scenarios (e.g. DSY2/DSY3, 90th percentile) where resilience under heatwaves is a priority, particularly for vulnerable occupants. Weather zones are now defined across 28 UK climate regions, accessed via the CIBSE Weather Data Selection Tool.

For installers and specifiers, this means cooling and ventilation strategies should be resilient to 2050s climate, not only current conditions, and AC plant sizing and refrigerant strategies should consider long‑term performance under repeated extreme events.

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Compliance checklist: Avoiding common TM59 mistakes

The TM59 overheating compliance checklist is designed to support QA and flag typical modelling errors and differences between TM59 and regulatory methods in England, Wales and Scotland.

Key points from the checklist include:

• Model set‑up:
  • Verify occupied hours: 3,672 hours for bedrooms, 1,989 hours for living rooms, kitchens and studies between May and September.
  • Check north orientation and solar angles carefully, using mapping tools where needed.
  • Confirm internal volumes and constructions are correctly represented, including floor‑to‑ceiling heights, voids and exposure types.
• Weather files:
  • Use the correct DSY1 2050s HIGH 50 file for the relevant zone.
  • Ensure cooling loads are zero unless mechanical cooling is explicitly specified.
• Category selection:
  • Use Category I for dwellings with thermally sensitive occupants (e.g. care homes, some social housing settings).
  • Use Category II for typical domestic dwellings.
• Regulation compliance differences:
  • Blinds should not be modelled as part of regulatory compliance unless external, interstitial or fixed shutters.
  • Trees must not be modelled as shading objects.
  • Security and noise constraints for bedroom windows must be reflected in night‑time opening assumptions.

The checklist also advises practitioners to liaise with acoustic consultants, check that mechanical solutions meet internal noise limits, and make sure overheating mitigations don’t conflict with other regulations.

From a Logic4training point of view, this reinforces why engineers should be comfortable with building regulations (Part O, Part F, Part L) and TM59 language, so they can understand what the modeller is doing and how that translates to real hardware and controls on site.

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Passive cooling and design: What TM59 expects

TM59 devotes significant space to passive summer temperature control as the preferred route to managing overheating.

Design hierarchy

The document explicitly references the Greater London Authority “cooling hierarchy”, which, in summary, is:

1. Reduce heat entering the building through orientation, shading, reflective surfaces and green infrastructure.
2. Minimise internal gains through efficient fabric, services and equipment.
3. Manage heat within the building using thermal mass and appropriate room volumes.
4. Provide passive ventilation (e.g. secure window openings, cross‑ventilation and night‑time purge).
5. Provide mechanical ventilation where necessary.
6. Provide active cooling only as a last resort, using the lowest carbon options.

TM59 gives practical commentary on:

• Avoiding excessive glazing, especially between north‑east and north‑west via south.
• Using external shading devices and solar control glazing with clear g‑values.
• Designing operable windows that provide ventilation without compromising safety or security.
• Making use of cross‑ventilation and stack effects in dual‑aspect flats and dwellings.
• Managing community heating and hot water gains through insulation, shorter pipe runs and careful plant room design.

Ceiling fans are recognised as an effective way to improve comfort when people are awake, and TM59 describes how to model their effect via uplifted thresholds rather than artificially reducing operative temperature.

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What this means for domestic AC and refrigerants

TM59 does not give specific equipment choices, but its principles shape where domestic AC and reversible heat pumps fit into the picture.

• Mechanical cooling should only be introduced at Stage 3, after robust passive and mechanical ventilation measures have been exhausted.
• Cooling systems should be controlled so windows and passive/mechanical ventilation are used whenever conditions allow.
• Systems should be energy efficient, targeted at spaces that genuinely need them, and easy to use so occupants don’t run them unnecessarily.

As demand for AC and cooling grows, refrigerant choice becomes critical. While TM59 doesn’t list specific refrigerants, the wider industry trend is clear:

• Higher‑GWP fluorinated gases must be managed carefully
• Low‑GWP hydrocarbons like R290 require competence in flammables.

This is exactly where Logic4training’s F‑Gas courses sit – equipping engineers to work safely and responsibly as TM59 continues to push design towards lower‑carbon solutions.

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Practical takeaways for installers and refurb specialists

Drawing TM59’s guidance into everyday practice, domestic engineers and refurb specialists can use a few simple habits to align their work with best practice:

• Ask design teams which TM59 criteria and weather files they are using and whether Category I or II applies.
• Treat shading, window operation, ventilation and hot water pipework insulation as integral parts of “cooling design”, not just fabric issues.
• Encourage clients to consider ceiling fans and improved ventilation as genuine comfort upgrades, not just add‑ons.
• If AC or cooling is specified, check that operating strategies respect TM59’s staged logic and avoid unnecessary running.
• Invest in training around refrigerant handling, including flammables, so you can confidently install and maintain systems that respond to TM59’s push for low‑carbon, resilient buildings.

This positions engineers as trusted advisors on comfort, safety and climate performance, not just equipment installers.

TM59 2026 gives the UK a clear, evidence‑based framework for understanding overheating risk in dwellings and for proving that designs genuinely protect occupants as summers get hotter. It pushes the industry towards passive measures first, then mechanical ventilation, and only finally towards cooling, all tested against realistic future weather and rigorously reported criteria.

For installers, refurb specialists and designers, this is both a challenge and an opportunity. Those who understand TM59’s criteria, three‑stage strategy and weather assumptions, and combine that knowledge with solid skills in cooling, refrigerants and heat pumps, will be best placed to help clients create homes that stay comfortable, safe and affordable to run in a warming climate.

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FAQs

In simple terms, what does TM59 2026 do?

It gives a prescriptive method for using dynamic thermal models to assess overheating risk in dwellings, with clear criteria, modelling assumptions and reporting requirements for designers and building services professionals.

Why is there a new bedroom overheating criterion?

TM59 2026 updates the bedroom criterion based on large‑scale research into sleep comfort, setting fixed night‑time thresholds and limiting the number of nights when mean bedroom temperature can exceed them, to protect sleeping comfort and health.

Which weather files must I use with TM59?

You must use the latest CIBSE DSY1 file for the relevant zone, based on the 2050s, high‑emission (RCP8.5), 50th percentile scenario, labelled ZoneReferenceDSY1 2050s HIGH 50 CIBSE v1.1 as a minimum for TM59 assessments.

How does TM59 link to Building Regulations?

Approved Document O and the Scottish Domestic Technical Handbook allow modelling routes that closely follow TM59, but some assumptions differ, which is why CIBSE provides a compliance checklist to highlight these differences and keep assessments coherent.

Where does air conditioning fit into TM59’s strategy?

AC and mechanical cooling appear only at Stage 3, after passive measures and mechanical ventilation have been fully explored; they should be targeted, efficient and controlled so that natural and mechanical ventilation are always used first when possible.

Where can I learn more about cooling and heat pump training?

Logic4training’s website includes dedicated sections for heat pump training and refrigeration/F‑Gas courses, covering cooling fundamentals, refrigerant safety and low‑carbon heating skills for domestic and small commercial work.